1. Technical Field
The present invention relates generally to apparatus and systems for communicating between a transmitter and a receiver. More particularly, the apparatus and systems relate to transmitting and receiving data through optical wireless or free-space optical communication. Specifically, the apparatus and systems of the present invention transmit and receive data with a common laser transmitter.
2. Background Information
Free-space optical links (FSO) (also called lasercomm) offer high-bandwidth and jam-resistant communications with a low probability of intercept and detection (LPI/LPD) between tactical edge platforms and users. However, these links require large telescopes, lasers and highly accurate pointing, acquisition and tracking systems to work. But in some cases, one end of the link cannot accommodate the size, weight and power (SWAP) of a lasercomm terminal and has lower data requirements than a conventional lasercomm link. An FSO link may still be desirable because of LPI/LPD, large SWAP of RF terminals even for these low data rates, and RF spectrum allocation may be limited. In these situations, a modulating retro-reflector (MRR) link is appropriate. An MRR couples a passive retro-reflector such as a corner-cube or cat's eye with an electro-optic modulator such as multiple-quantum-well (MQM) modulator. As shown in FIG. 1, typically in an MRR link, a conventional, actively pointed, lasercomm terminal 104 (e.g., interrogator) on one end interrogates an MRR 102 on the other end of the link with a continuous wave (CW) laser beam 106. This beam 106 is passively retroreflected back to the interrogator as a retroreflected beam 108 with a data signal imposed on it by the modulator 102. The technique is appropriate for unattended sensors or disadvantaged users. The link falls of as (distance)−4. Data rates of up to 10 Mbps is possible for corner cube retro-reflector and 10's Mbps for cat's eye retro-reflector.
Naval Research Laboratory (NRL) has been actively working for over a decade in developing MQW modulators and MRR links and they have disclosed their work in many journals and conference papers, for example: (i) G. Charmaine Gilbreath, William S. Rabinovich, Rita Mahon, L. Swingen, Fun Oh, Timothy Meehan, and Peter Goetz, “Real-Time 1550 nm Retromodulated Video Link”, Proceedings of the 2003 IEEE Aerospace Conference, Paper No. 1560; (ii) W. S. Rabinovich, R. Mahon, P. G. Goetz, E. Waluschka, D. S. Katzer, S. C. Binari, and G. C. Gilbreath, “A Cat's Eye Multiple Quantum-Well Modulating Retro-Reflector, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, pp. 461-63, 2003; and (iii) W. S. Rabinovich, R. Mahon, H. R. Burris, G. C. Gilbreath, P. G. Goetz, C. I. Moore, M. F. Stell, M. J. Vilcheck, J. L. Witkowsky, L. Swingen, M. R. Suite, E. Oh, and J. Kaplow, “Free-space optical communication link at 1550 nm using multiple-quantum-well modulating retro-reflectors in a marine environment” Optical engineering, Vol. 44, p. 056001-1, 2005, the contents of which are incorporated herein by reference. By their very nature as discussed above and shown in FIG. 1, MRR links are either half duplex or the data flows only from the MRR to the interrogator unless another optical wavelength from another transmitter is used to transmit data in the other direction, i.e., from the interrogator 104 to the MRR terminal 102. When data is transmitted in both directions at different wavelengths, an optical filtering will be needed at both ends in addition to another optical transmitter at the interrogator 104. This will increase the size weight and power (SWAP) and cost. A better FSO lasercomm system is needed.